Successful marrow transplantation requires not only genetically compatible donors but the elimination of normal or malignant hematopoietic cells of the recipient without destruction of other vital tissues. Procedures for irradiating experimental animals were recommended by the International Commission on Radiological Units and Measures. The investigation of physical factors affecting successful marrow transplantation in inbred mice has demonstrated that the ICR recommendations were obsolete and other basic concepts of radiation biology were invalid. Exposure-rate effects and exposure-rate + absorption significantly altered the repair capabilities of normal and malignant hematopoietic cells. Repair was initiated only during exposure and the pattern of repair required only 10 min. to become established. Successful repair of hematopoietic cells interfered with establishment of the marrow graft resulting in partial chimerism or complete re-establishment of the marrow graft resulting in partial chimerism or complete reversal to recipient genotype. Significant differences in results occurred with simultaneous 2 direction, dorsal, ventral and reciprocal alternate dorsal + ventral and ventral + dorsal exposures to X- rays. Similar effects are being found using gamma radiation from Ces 137. Cellular repair has usually been investigated using tissue culture system rather than intact animals which gave new incite into this phenomenon. There must be changes in the way physical factors are reported when dealing with biological systems to insure reproducibility of experimental data. In addition the universal practice of converting roentgens as calibrations in air to Grays as absorbed tissue doses should be abandoned in all cases exact measurements are impractical.